Objectives: (1) To study the correlation among conventional clinical and laboratory parameters and the relation between the number of lymphocytes and neutrophils (L/N) in cell suspensions from peripheral blood of patients with juvenile idiopathic arthritis (JIA). (2) To evaluate the L/N relation of RA patients after an 8 year follow-up period. Methods: Fifty-one JIA patients (25 female, disease course: 19 systemic, 15 polyarticular, 17 pauciarticular) were enrolled in the study. To measure the L/N relation, we used Boyum's method: The leucocyte separation was done by centrifugation of peripheral blood on Ficoll-Hypaque (FH) gradient, and the number of lymphocytes, monocytes, and neutrophils in 500 cells was determined. The following clinical and laboratory parameters were evaluated: disease activity, number of active and limited joints, functional capacity, erythrocyte sedimentation rate (ESR), and C reactive protein (CRP). Twenty-four healthy children were used as controls. We also studied 13/51 patients from our Pediatric Rheumatology Unit who had been evaluated by the same method 8 years before. Results: We observed the lowest L/N relation in patients with active disease, especially those with polyarticular course. A statistical con-elation was also observed with the acute-phase reactants (ESR and CRP, p < 0.05). The majority of patients who had presented a low L/N relation at the first evaluation (8 years before) had a worse outcome. Conclusion: The measure of L/N relation from peripheral blood could be used as an auxiliary tool in the assessment of the activity and outcome of JIA patients, especially at disease onset.